use reduce to serialize claimJob attempts

src/worker.js

@@ -117,7 +117,7 @@ export default class Job extends events.EventEmitter {
     this.debug(`Starting job ${job._id}`);
 
     const workerOutput = new Promise((resolve, reject) => {
-      resolve(this.workerFn(job._source.payload));
+      resolve(this.workerFn.call(null, job._source.payload));
 
       setTimeout(function () {
         reject(new WorkerTimeoutError({
@@ -177,41 +177,49 @@ export default class Job extends events.EventEmitter {
   }
 
   _claimPendingJobs(jobs) {
+    if (jobs.length === 0) return;
+
     this._stopJobPolling();
     let claimed = false;
 
-    return Bluebird.mapSeries(jobs, (job) => {
-      if (claimed) return false;
+    return jobs.reduce((chain, job) => {
+      return chain.then((claimedJob) => {
+        // short-circuit the promise chain if a job has been claimed
+        if (claimed) return claimedJob;
 
-      return this._claimJob(job)
-      .then((claimedJob) => {
-        if (claimedJob !== false) {
-          claimed = true;
-          return claimedJob;
-        }
-      })
-      .catch((err) => {
-        this.debug('Failed to claim outstanding jobs', err);
-        this.emit('error', err);
-        this.queue.emit('worker_error', {
-          id: this.id,
-          type: this.type,
-          err
+        return this._claimJob(job)
+        .then((claimResult) => {
+          if (claimResult !== false) {
+            claimed = true;
+            return claimResult;
+          }
         });
-        throw err;
       });
+    }, Promise.resolve())
+    .catch((err) => {
+      this.debug('Failed to claim outstanding jobs', err);
+      this.emit('error', err);
+      this.queue.emit('worker_error', {
+        id: this.id,
+        type: this.type,
+        err
+      });
+      throw err;
     })
-    .then((mappedJobs) => mappedJobs.filter(Boolean))
-    .then((claimedJobs) => {
-      if (claimedJobs.length !== 1) {
+    .then((claimedJob) => {
+      if (!claimedJob) {
         this.debug(`All ${jobs.length} jobs already claimed`);
         return;
       }
-      const job = claimedJobs[0];
-      this.debug(`Claimed job ${job._id}`);
-      return this._performJob(job);
+      this.debug(`Claimed job ${claimedJob._id}`);
+      return this._performJob(claimedJob);
     })
-    .finally(() => this._startJobPolling());
+    .then(() => this._startJobPolling())
+    .catch((err) => {
+      this.debug('Error claiming jobs', err);
+      this.emit('error', err);
+      this._startJobPolling();
+    });
   }
 
   _getPendingJobs() {